Small-sized electronic devices, such as notebook-type personal computers, mobile phones, personal data assistants (PDAs), and the like, often use lithium ion secondary batteries as the power supplies. Furthermore, in recent years, lithium ion secondary batteries are also used in automobile applications. As the use of lithium ion secondary batteries becomes more widespread, requirements for performance and safety of a lithium ion secondary battery (hereinafter also referred to simply as battery) are increasing. Then this kind of batteries are usually used repeatedly by charging and discharging, which may cause an electrolyte solution to leak out by rising a pressure inside the battery as a result of volume change of an electrode and heat generation due to a repeat of charging and discharging operations. In such a case, not only deterioration occurs in battery characteristics, but a problem also exists on a hazard such as firing and corrosion of the device. For example, when the electrolyte solution is used for a lithium ion secondary battery, since electrolyte solution is organic-based, and thus it dislikes water extremely, this type of lithium ion secondary battery is required to have high sealing performance such that water is completely prevented from entering inside of the battery, and electrolyte solution is also completely prevented from leaking out.
For example, a lithium ion secondary battery includes a metal housing to seal a power generation element thereof, and electrical insulation is required between a positive electrode terminal and a negative electrode terminal in order to prevent short circuiting between the positive electrode and the negative electrode. For electrical insulation between the positive and the negative electrodes, and for sealing, a gasket made of electrically insulating material is usually used on an opening of the metal housing that houses the power generation element. As the electrically insulating material, an electrically insulating gasket made of resin is known to be used (see, e.g., Patent Literature 1).
In order to improve the sealing by such an insulating gasket, use in combination of an insulating gasket and a sealing material is also suggested (see, e.g., Patent Literatures 2 to 4, etc.). The sealing material is applied on the insulating gasket or on the metal housing, and the insulating gasket is then attached to the metal housing, leading to improving the sealing performance between the insulating gasket and the metal housing.
Examples of such a sealing material include pitch-based materials, such as coal tar, asphalt, etc., as well as materials obtained by adding a polymer as a modifier to a pitch-based material (see, e.g., Patent Literatures 5 to 9, etc.). In addition, examples other than pitch-based materials include butyl rubber (see, e.g., Patent Literature 10, etc.), polyolefin-based adhesive (see, e.g., Patent Literature 11, etc.), polyvinylidene fluoride resin (see, e.g., Patent Literature 12, etc.), and the like.